The contribution of plant biology to the concept of virus (1886-1917)

Abstract

Between 1860 and 1880 the so-called "theory of the infective germ", which stated in final way that every infectious disease was produced by a living pathogen agent, achieved great consent. The criteria of determining the presence of infectious pathogens (fungi, bacteria, protozoa) were established by "Koch's postulate", a set of experimental procedures conceived for isolating and determining single pathogens. In the last quarter of the 19th century became however evident that the agents of severe infectious diseases could not be identified through the "postulates". These agents could not be seen in light microscopy nor cultured in vitro but could pass through the thin pores of filters which hold back cellular micro-organisms. This last characteristic became a selective method to recognise these peculiar agents, from then named "filterable viruses". Most microbiologists considered the filterable viruses as living micro-organisms because of their extraordinary capacity of in vivo proliferation, and the impossibility of pointing out their structures was due to limits of the experimental techniques. Between the end of the 19th century and 1917, four plant biologists suggested that the filterable viruses were complex chemical substances rather than cellular microorganisms. Their contribution, not appreciated by the contemporary colleagues, laid the foundation of the modern concept of virus.